You can now mark your shouts as a Spoiler Alert when commenting on anything. Pretty simple to do so, just check the box right above the shout button.
When viewing the site, anywhere that displays shouts with spoilers will now look like this. Just click on it to reveal the shout.
If you want to automatically display any spoilers (this is essentially the view before this new feature was added), you can do so on your Viewing trakt settings page (http://trakt.tv/settings/viewing). Also note you can hide the red badge that indicates new shouts were found.
Note: The API methods have also been updated so you can flag spoilers there too. Any methods that get shouts will have a new spoiler property. Check the updated docs for more info.